Piezoelectric actuator

ABSTRACT

A piezoelectric actuator in which at least on piezoelectric element is present for subjected an actuating element to a tensile force or compressive force. In addition, stabilizing elements are mounted parallel to the piezoelectric element with a flexible intermediate layer located between the elements. The piezoelectric element and the stabilizing elements have a great length in the effective direction (Z axis) in proportion to their width transversely to the effective direction (X, Y direction).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 00/01671 filed onMay 24, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a piezoelectric actuator, for instance foractuating a mechanical component such as a valve or the like.

The invention relates to a piezoelectric actuator, for instance foractuating a mechanical component such as a valve or the like, inaccordance with the generic characteristics of the preamble to the mainclaim.

2. Description of the Prior Art

The aforementioned piezoelectric actuators are often used in thepositioning of valves. Among other factors, it must be considered herethat their stroke capacity for actuating a valve tappet, for instance,is relative slight for a comparatively great force. To increase theuseful stroke, it is therefore known to provide a mechanical orhydraulic travel booster. Such mechanical or hydraulic travel boostingsystems, however, entail greater effort and therefore greater expense aswell.

It is widely known that by utilizing the so-called piezoelectric effect,a piezoelectric element can be constructed from a material with asuitable crystalline structure. When an external electrical voltage isapplied, a mechanical reaction of the piezoelectric element takes place,which depending on the crystalline structure and the regions where theelectrical voltage is applied causes a compression or tension in apredeterminable direction.

The aforementioned piezoelectric actuators are often used in thepositioning of valves. Among other factors, it must be considered herethat their stroke capacity for actuating a valve tappet, for instance,is relative slight for a comparatively great force. To increase theuseful stroke, it is therefore sometimes usual to provide a mechanicalor hydraulic travel booster. Such mechanical or hydraulic travelboosting systems, however, entail greater effort and therefore greaterexpense as well.

SUMMARY OF THE INVENTION

The piezoelectric actuator described at the outset advantageously has atleast one piezoelectric element, which is suitable for imposing atensile force or compressive force on an actuating element. According tothe invention, stabilizing elements are provided, which are disposedparallel to the piezoelectric element with a flexible intermediate layerlocated between the elements, wherein the piezoelectric element and thestabilizing elements have a great length in the effective direction (Zaxis) in proportion to their width transversely to the effectivedirection (X, Y direction). An advantageous order of magnitude would forinstance be a ratio of length (Z direction) to width (X, Y direction) isapproximately 5:1 to 50:1.

In a first advantageous embodiment, the stabilizing elements are ofsteel and are held between a base or support plate fastened firmly inthe housing of the piezoelectric actuator and a fixation edge in thehousing. The housing here is as a rule also made from steel. Thepiezoelectric element is held between the base plate and a spring platewhich, via a prestressing spring, likewise rests on the housing andguides the actuating element.

With the invention, a long, narrow piezoelectric actuator is created ina simple way that is mechanically relatively invulnerable, for instanceto vibration when used in the engine compartment of a motor vehicle.Because of the long stroke resulting from the long, narrow design, astroke boost can be dispensed with, and in principle, tensile forces orcompressive forces can be generated by the piezoelectric actuator.

Because a flexible intermediate layer, for instance of plastic such as apolymer or the like, is mounted between the stabilizing elements and thepiezoelectric element, a longitudinal motion, which represents arelative motion between the piezoelectric element and the stabilizingelement, can be allowed. An oscillating motion of the piezoelectricelement in the X or Y direction can be avoided, however. It is thuspossible in a simple way to prevent bending tensions in thepiezoelectric element that could possibly lead to the destruction of thepiezoelectric actuator.

The aforementioned concrete embodiment furnishes downward-orientedcompressive forces and is not temperature-compensated. In a secondembodiment, the piezoelectric element and the stabilizing elements areof ceramic materials, which have essentially the same coefficients oftemperature expansion, so that this embodiment istemperature-compensated. The stabilizing element is here as well heldbetween a base plate and a fixation edge in the housing, but the baseplate rests on the housing via a spring.

In this last embodiment, the stabilizing element is mechanically coupledto the piezoelectric element in such a way that the temperature-dictatedexpansions of the piezoelectric element and of the stabilizing elementscancel one another out in the effective direction in such a way that theactuating element remains in its position, or in other words nozero-point drift occurs. The piezoelectric element is held between thebase plate and a spring plate, which via a prestressing spring likewiserests on the housing and guides the actuating element. Here the force ofthe prestressing spring must be substantially greater than that of thespring on the base plate, so that the different temperature expansionsbetween the housing and the material comprising the piezoelectricelement are compensated for via the spring. Additional provisions fortemperature compensation, as was often provided until now by hydrauliccoupling, are no longer necessary here.

In the invention, the piezoelectric element can be constructed oftransversely stacked piezoelectric layers and thus exerts a compressiveforce on the actuating element, or of longitudinally stackedpiezoelectric layers and thus exerts a tensile force on the actuatingelement.

In other advantageous embodiments, the stabilizing element comprisespiezoelectric layers, each located perpendicular to the layeredstructure of the piezoelectric element, which piezoelectric layers aretriggered with a voltage in the same way as the piezoelectric element.With this embodiment, in addition to the temperature compensation, theuseful stroke is also lengthened by means of the additional stroke ofthe stabilizing elements. The contacting of the piezoelectric layers canbe located in the intermediate layer or outside it.

In another advantageous embodiment of the invention, two piezoelectricelements are disposed symmetrically to a tension rod, acting as theactuating element, surrounded by the intermediate layer in the housingof the piezoelectric actuator. The piezoelectric elements here are heldbetween a support plate, connected to the tension rod, and a fixationedge in the housing, and the support plate rests on the housing via aspring.

These and other characteristics of preferred refinements of theinvention will become apparent from the description and the drawings;the individual characteristics, each alone or a plurality of them in theform of subsidiary combinations, can be realized in the embodiment ofthe invention and in other fields and can represent both advantageousand intrinsically patentable embodiments for which patent protection ishere claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the piezoelectric actuator of the inventionwith a narrow design, for instance for positioning a valve, will beexplained in conjunction with the drawings, in which:

FIG. 1, a section through a non-temperature-compensated piezoelectricactuator with steel stabilizing elements;

FIG. 2, a detail taken along a section line A—A of FIG. 1, with a firstcontacting option for the piezoelectric element;

FIG. 3, a detail taken along a section line A—A of FIG. 1, with a secondcontacting option for the piezoelectric element;

FIG. 4, a section through a temperature-compensated, transverselystacked piezoelectric element of the piezoelectric actuator, with aceramic stabilizing element;

FIG. 5, a section through a temperature-compensated, longitudinallystacked piezoelectric element of the piezoelectric actuator, with aceramic stabilizing element;

FIG. 6, a detail taken along a section line A—A of FIG. 5, with a firstcontacting option for the piezoelectric element;

FIG. 7, a detail taken along a section line A—A of FIG. 5, with a secondcontacting option for the piezoelectric element; and

FIG. 8, a section through a non-temperature-compensated piezoelectricactuator with two piezoelectric elements disposed one on either side ofa tension rod.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a piezoelectric actuator 1 is shown that has a piezoelectricelement 2, which in a manner known per se is constructed ofpiezoelectric sheets of a quartz material with a suitable crystallinestructure, so that by utilizing the so-called piezoelectric effect uponapplication of an external electrical voltage to electrodes 3 and 4, amechanical reaction of the piezoelectric actuator 1 is effected, in theform of a useful force F_(u). The exemplary embodiment in FIG. 1 is nottemperature-compensated and furnishes a compressive force F_(u).

The relatively long, slender piezoelectric element 2 is pressed by itsupper end against a base or support plate 8 by means of a prestressingspring 6 and a spring plate 7, disposed above this spring, on the lowerend of the piezoelectric element. The support plate 8 is braced withstabilizing elements 9, disposed symmetrically on both sides of thepiezoelectric element 2, which are braced at the top and bottom on thehousing 10 of the piezoelectric actuator 1. Between the stabilizingelements 9 and the piezoelectric element 2 is a flexible plastic, suchas a polymer, located longitudinally in the form of an intermediatelayer 11. The task of the flexible intermediate layer 11 is to allow alongitudinal motion, that is, the relative motion between thepiezoelectric element 2 and the stabilizing elements 9, but to preventan oscillating motion of the piezoelectric element in the X or Ydirection.

From FIGS. 2 and 3, possibilities for a connection of the contacts 3 and4 of the piezoelectric element 2 are suggested; these can be effected ineither the X or Y direction and can thus be located either in theintermediate layer 11 or outside it.

A second exemplary embodiment of a piezoelectric actuator 20 is shown inFIG. 4; here, components functioning the same are identified by the samereference numerals as in FIG. 1. In the arrangement of FIG. 4, there isone piezoelectric element 21, and there are stabilizing elements 22 thatare made from a ceramic material with approximately the same coefficientof temperature expansion as the piezoelectric element 21. The supportplate 8 is prestressed here in the housing 10 via a spring 23; theprestressing force of the spring 23 must be substantially greater thanthat of the prestressing spring 6, so that the different temperatureexpansions between the housing 10 and the piezoelectric element 21 canbe compensated for via the spring 23. However, it is also possible toomit the spring 23. Then the piezoelectric element 21 would be heldtogether by the upper support plate 8, analogously to what FIG. 1 shows,and optionally also by a lower support plate. The upper support plate 8would, as in FIG. 1, rest on a shoulder of the housing 10. The spring 23prevents the stabilizing element 22 from being subject to tensile stressthat can be caused by the prestressing of the piezoelectric element.This prestressing is accomplished with the aid of the spring 6.

An actuation of the piezoelectric actuator 20, in this exemplaryembodiment as well, leads to an axial expansion of the piezoelectricelement 21 and thus to a compressive force F_(u) counter to theprestressing of the prestressing spring 6. Since here as well thepiezoelectric element 21 and the stabilizing elements 22 haveessentially the same coefficients of temperature expansion, thetemperature-caused expansions of the piezoelectric element 21 and of thestabilizing elements 22, in the proposed mechanical mounting, cause theinfluences of the two elements 21 and 22 to be cancelled out in theeffective direction. Thus the actuating element, connected to the springplate 7 of the piezoelectric element 21, can remain in its position.

A third exemplary embodiment of a piezoelectric actuator 30 is shown inFIG. 5; once again, components that function the same are provided withthe same reference numerals as in FIG. 1 or FIG. 4. In the arrangementof FIG. 5, in contrast to the disposition of FIG. 4, only apiezoelectric element 31 with longitudinally stacked piezoelectriclayers is provided. An actuation of the piezoelectric actuator 30 inthis exemplary embodiment leads to an axial shortening of thepiezoelectric element 31 and thus to useful tensile force F_(u) thatacts on the actuating element.

In FIGS. 6 and 7, once again possibilities for a connection of thecontacts 3 and 4 of the piezoelectric element 31 are suggested; thesecan again be effected with suitably oriented piezoelectric layers ineither the X or Y direction and can thus be located either in theintermediate layer 11 or outside it.

From FIG. 8, a further exemplary embodiment of a piezoelectric actuator40 can be seen; once again, the components functioning the same areprovided with the same reference numerals as in FIG. 1, FIG. 4 or FIG.5. In the arrangement of FIG. 8, two piezoelectric elements 41 and 42are disposed symmetrically to a tension rod 43, which represents theactuating element. The piezoelectric elements 41 and 42 and the tensionrod 43 are disposed, surrounded by the intermediate layer 11, in thehousing 10 of the piezoelectric actuator 40. The piezoelectric elements41 and 42 here are furthermore held between a support plate 44,connected to the tension rod, and via the spring 23 on an upper fixationedge in the housing 10 and a lower fixation edge in the housing 10. Thisarrangement furnishes a tensile force as the force F_(u) and is nottemperature-compensated.

The foregoing relate to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A piezoelectric actuator comprising a housing (10), in which at least one piezoelectric element (2; 21; 31; 41; 42) for subjecting an actuating element to a tensile force or compressive force is disposed and is guided longitudinally movably by means of a flexible intermediate layer (11), and stabilizing elements (9, 22), disposed parallel to this piezoelectric element (2; 21; 31; 41, 42), to prevent bending tensions in the piezoelectric element (2; 21; 31; 41; 42) said intermediate layer (11) being located between the piezoelectric element (2; 21; 31; 41; 42) and the stabilizing elements (9, 22), the ratio of the length of the piezoelectric element (2; 21; 31; 41; 42) and the stabilizing elements (9, 22) in the effective direction (Z axis) to the width transversely to the effective direction (X, Y direction) is from 5:1 to 50:1.
 2. The piezoelectric actuator of claim 1, wherein the stabilizing elements (9) are of steel and are held between a base or support plate (8) fastened firmly in the housing (10) of the piezoelectric actuator (1) and a fixation edge in the housing (10), and wherein the piezoelectric element (2; 21; 31; 41; 42) is held between the base plate (8) and a spring plate (7) which, via a prestressing spring (6), likewise rests on the housing (10) and guides the actuating element.
 3. The piezoelectric actuator of claim 1, wherein the piezoelectric element (21; 31) and the stabilizing elements (22) are of ceramic materials, which have essentially the same coefficients of temperature expansion, and the stabilizing elements (22) are held between a base or support plate (8) and a fixation edge in the housing (10), wherein the piezoelectric element (21; 31) is held between the base plate (8) and a spring plate (7), which via a prestressing spring (6) likewise rests on the housing (10) and guides the actuating element, and wherein the stabilizing element (22) is coupled mechanically with the piezoelectric element (21; 31) in such a way that the temperature-dictated expansions of the piezoelectric element (21; 31) and of the stabilizing element (22) cancel one another out in the effective direction in such a way that the actuating element remains in its position.
 4. The piezoelectric actuator of claim 3, wherein the base plate (8) rests on the housing (10) via a spring (23).
 5. The piezoelectric actuator of claim 2, wherein the piezoelectric element (21) is constructed of transversely stacked piezoelectric layers and thus exerts a compressive force on the actuating element.
 6. The piezoelectric actuator of claim 3, wherein the piezoelectric element (21) is constructed of transversely stacked piezoelectric layers and thus exerts a compressive force on the actuating element.
 7. The piezoelectric actuator of claim 4, wherein the piezoelectric element (21) is constructed of transversely stacked piezoelectric layers and thus exerts a compressive force on the actuating element.
 8. The piezoelectric actuator of claim 2, wherein the piezoelectric element (21) is constructed of longitudinally stacked piezoelectric layers and thus exerts a tensile force on the actuating element.
 9. The piezoelectric actuator of claim 3, wherein the piezoelectric element (21) is constructed of longitudinally stacked piezoelectric layers and thus exerts a tensile force on the actuating element.
 10. The piezoelectric actuator of claim 4, wherein the piezoelectric element (21) is constructed of longitudinally stacked piezoelectric layers and thus exerts a tensile force on the actuating element.
 11. The piezoelectric actuator of claim 5, wherein the stabilizing elements (22) comprise piezoelectric layers, each located perpendicular to the layered structure of the piezoelectric element (21; 31), which piezoelectric layers are triggered with a voltage in the same way as the piezoelectric element (21; 31).
 12. The piezoelectric actuator of claim 6, wherein the stabilizing elements (22) comprise piezoelectric layers, each located perpendicular to the layered structure of the piezoelectric element (21; 31), which piezoelectric layers are triggered with a voltage in the same way as the piezoelectric element (21; 31).
 13. The piezoelectric actuator of claim 7, wherein the stabilizing elements (22) comprise piezoelectric layers, each located perpendicular to the layered structure of the piezoelectric element (21; 31), which piezoelectric layers are triggered with a voltage in the same way as the piezoelectric element (21; 31).
 14. The piezoelectric actuator of claim 8, wherein the stabilizing elements (22) comprise piezoelectric layers, each located perpendicular to the layered structure of the piezoelectric element (21; 31), which piezoelectric layers are triggered with a voltage in the same way as the piezoelectric element (21; 31).
 15. The piezoelectric actuator of claim 9, wherein the stabilizing elements (22) comprise piezoelectric layers, each located perpendicular to the layered structure of the piezoelectric element (21; 31), which piezoelectric layers are triggered with a voltage in the same way as the piezoelectric element (21; 31).
 16. The piezoelectric actuator of claim 10, wherein the stabilizing elements (22) comprise piezoelectric layers, each located perpendicular to the layered structure of the piezoelectric element (21; 31), which piezoelectric layers are triggered with a voltage in the same way as the piezoelectric element (21; 31). 